How to improve pearlite content

The high-temperature normalizing of ductile iron is to heat the workpiece to the upper limit of critical temperature +(30 ~ 50℃), keep the temperature for a period of time, make the matrix completely transform into austenite, homogenize the austenite, and then air-cool. Normalization heating temperature is generally 900 ~ 940℃. Too high temperature can not only increase the amount of pearlite, but also cause the austenite grains to be coarse, so that the amount of carbon dissolved in austenite is too much. When cooling, it is easy to precipitate reticular secondary cementite at the grain boundary, which makes the strength and toughness of ductile iron castings worse. Increasing the cooling rate of normalizing can increase the amount of pearlite. When the austenitizing temperature is higher than this temperature, the microstructure of ductile iron will undergo a series of changes. The equilibrium carbon content of austenite increases with the increase of temperature. Carburizing of austenite requires the dissolution and diffusion of graphite carbon, so with the passage of time, the carbon content of austenite gradually reaches equilibrium at this temperature. Increasing the heating speed and shortening the holding time can make the actual carbon content of austenite lower than the equilibrium carbon content at this temperature and obtain low-carbon austenite. If the heating speed is fast, the original structure will affect the actual carbon content of austenite.